Evaluation of Reducibility and Reduction Strength of Burden Materials Taken from the LKAB Experimental Blast Furnace

摘要

Samples taken from excavations of the experimental blast furnace show similar degree of reduction, despite large differences in reducibility values (R40) in reducibility tests. The reason for this deviation is that the reducing conditions differ between laboratory tests and the blast furnace. The ISO 4695 reducibility test is developed to distinguish between different materials, using quite high test temperature and high amount of carbon monoxide gas, making the tested material limiting on reducibility. In a modern blast furnace, though, reduction times are longer and with high amounts of injected reductant, providing generous reducing conditions in terms of reducing gas hydrogen contents, no discrimination of less reducible materials is found. Therefore, R40 reducibility values from the standardized ISO 4695 test do not show how a specific burden material would behave in a blast furnace, and should not be used in selecting or ranking different pellet types for the blast furnace operation. However, since the test is developed to be decisive it can still be used as a quality, or rather consistency, parameter in pellet production. In spite of very different chemical composition, and varying results in the ITH reduction strength test, most materials of this investigation follow a general trend in reduction strength during reduction in the blast furnace, First there is a minimum in reduction strength at around 10% reduction degree, corresponding to the magnetite to hematite reduction stage. During further reduction some of the reduction strength is restored, in the end giving high reduction strength values when iron starts to form. The only materials deviating from this general trend were the Basic 3 pellet, which had considerably low raw material reduction strength value, and the sinter. As for the reducibility test, the ITH reduction strength test for raw materials is not representative for what happens in a blast furnace. Only if a material gets a very low reduction strength value, it can be assumed that the same could occur in a blast furnace. Therefore, also the ITH reduction strength test should not be used in ranking pellet types, but could well be used as a consistency control in pellet production.